Mallor's reign saw events that would have a huge impact on Middle-earth's history, but whose importance weren't fully recognized at the time. The first of these was the arrival across the Misty Mountains of a strange race of beings: similar to Men, but far shorter and with hairy, bootless feet. The Dúnedain called them periannath, "halflings", and they were the first Hobbits to enter the western lands.

The second historic event was the descent of a strange shadow on Greenwood the Great. This was wholly mysterious at first, but before the end of Mallor's reign, the Wise had discovered that its source was at Dol Guldur in the southern part of the forest. It was not discovered until long after Mallor's time that his reign had seen the return of Sauron himself, the first move in a game that would end in the War of the Ring nearly two thousand years in the future.

It was also during Mallor's reign that the Istari - the Wizards - arrived in the north of Middle-earth, soon after the shadow had fallen on Greenwood the Great. At this time, though, even the Wise did not suspect that it might be Sauron returned.

Mallor ruled Arthedain for eighty-one years, and was succeeded by his son, Celepharn, upon his death in 1029.